FIG. 1 shows a cross section of an exemplary conventional light sensor 102, which is essentially a single photodiode, also referred to as a photodetector. The photodetector 102 includes an N+ region 104, which is heavily doped, and a P− region 106 (which can be a P− epitaxial region), which is lightly doped. All of the above is likely formed on a P+ or P++ substrate 108, which is heavily doped. It is noted that FIG. 1 and the remaining FIGS. are not drawn to scale.
Still referring to FIG. 1, the N+ region 104 and P− region 106 form a PN junction, and more specifically, a N+/P− junction. This PN junction is reversed biased, e.g., using a voltage source (not shown), which causes a depletion region 110 around the PN junction. When light 112 is incident on the photodetector 102 (and more specifically on the N+ region 104), electron-hole pairs are produced in and near the diode depletion region 110. Electrons are immediately pulled toward N+ region 104, while holes get pushed down toward P− region 106. These electrons (also referred to as carriers) are captured in N+ region 104 and produce a measurable photocurrent, which can be detected, e.g., using a current detector (not shown). This photocurrent is indicative of the intensity of the light 112, thereby enabling the photodetector to be used as a light sensor. The portion of the photodetector 102 that produces a photocurrent in response to light incident on the photodetector can be referred to as the photodetector sensor region, or simply as the sensor region.
Photodetectors, such as but not limited to the exemplary photodetector 102, can be used as ambient light sensors (ALSs), e.g., for use as energy saving light sensors for displays, for controlling backlighting in portable devices such as mobile phones and laptop computers, and for various other types of light level measurement and management. For more specific examples, ambient light sensors can be used to reduce overall display-system power consumption and to increase Liquid Crystal Display (LCD) lifespan by detecting bright and dim ambient light conditions as a means of controlling display and/or keypad backlighting. Without ambient light sensors, LCD display backlighting control is typically done manually whereby users will increase the intensity of the LCD as the ambient environment becomes brighter. With the use of ambient light sensors, users can adjust the LCD brightness to their preference, and as the ambient environment changes, the display brightness adjusts to make the display appear uniform at the same perceived level; this results in battery life being extended, user eye strain being reduced, and LCD lifespan being extended. Similarly, without ambient light sensors, control of the keypad backlight is very much dependent on the user and software. For example, keypad backlight can be turned on for 10 seconds by a trigger which can be triggered by pressing the keypad, or a timer. With the use of ambient light sensors, keypad backlighting can be turned on only when the ambient environment is dim, which will result in longer battery life. In order to achieve better ambient light sensing, ambient light sensors preferably have a spectral response close to the human eye response and have excellent infrared (IR) noise suppression. Such a spectral response is often referred to as a “true human eye response” or a “photopic response”.
FIG. 2 shows an exemplary spectral response of a photodetector (e.g., the photodetector 102) without any spectral response shaping, e.g., using a filter covering the detector. FIG. 3 illustrates the spectral response of a typical human eye (also known as the “true human eye response” or the “photopic response”, as mentioned above). As can be appreciated from FIGS. 2 and 3, a potential problem with using a photodetector as an ambient light sensor is that it detects both visible light and non-visible light, such as infrared (IR) light, which starts at about 700 nm. By contrast, notice from FIG. 3 that the human eye does not detect IR light. Thus, the response of a photodetector can significantly differ from the response of a human eye, especially when the light is produced by an incandescent light, which produces large amounts of IR light. This would provide for significantly less than optimal adjustments if the photodetector were used as an ambient light sensor, e.g., for adjusting backlighting, or the like. Accordingly, various techniques have been attempted to provide light sensors that have a spectral response closer to that of a human eye, so that such light sensors can be used, e.g., for appropriately adjusting the backlighting of displays, or the like. Some of these techniques involve covering such light sensor with optical filters.
Typically, organic based optical filters cannot be used to provide a true human eye response, because organic based optical filters do not sufficiently absorb and/or reflect infrared light. Rather, non-organic filters, such as filters made of dielectric optical coatings, are generally preferred because they provide better performance. Such dielectric optical coatings, which are made from stacks of various dielectric films, are conventionally expensive to implement. This is in part because they are typically patterned using a photoresist lift-off in a chemical solvent bath, which is typically costly due to the relatively long residence time (i.e., soak duration) in the photoresist solvent bath, and due to the relatively narrow process margin. Alternatively, acoustic cleaning can be used, which is also typically costly.